Photocouplers are applied to communication circuits for transmission and reception of signals between an outdoor unit of an air conditioner and an indoor unit thereof. The photocouplers are used for electrical insulation between the outdoor unit and the indoor unit. Such a photocoupler includes a light emitting diode and a phototransistor, and is used for pulse signal transfer.
Phototransistors need a longer switching time from an ON state to an OFF state than a switching time from the OFF state to the ON state because of a mirror effect. For ease of understanding, the photocoupler is assumed in which an input of a high-level signal causes the light emitting diode to emit light and then the phototransistor to turn on, thereby resulting in an output of a high-level signal. When a pulse pair having a duty ratio of 50% is input to this photocoupler, a positive pulse width of the pulse pair to be output by this photocoupler increases, and a negative pulse width decreases accordingly. Thus the output pulse pair has a duty ratio that is greater than 50%. The use of the photocoupler thus causes a distortion of waveform of a transferred signal and also causes a change in the duty ratio thereof. The smaller pulse width increases the rate of change in the pulse width.
When the communication speed between the outdoor unit and the indoor unit is low, the output waveform of the photocoupler is not consistent with the input waveform, but this inconsistency does not interfere with transfer of information. However, the higher communication speed increases the distortion of the pulse to be transferred, eventually causing a transmission error. Hence, the communication speed is restricted.
In addition, the switching time of the phototransistor varies depending on the individual difference of the photocoupler at the time of manufacturing, a change in environmental condition like a surrounding temperature, aged deterioration, or the like. Thus photocouplers vary in response speed. Still further, uniform prediction and addressing of a change in pulse width originating from a photocoupler are difficult.
Yet still further, a communication line for transferring signals has a stray capacitance. This stray capacitance causes a delay in transferring signals, and causes a distortion of waveform thereof.
Moreover, when the communication line is set to be an open state (high impedance state), the voltage induced by the stray capacitance increases. The effect of this induced voltage restricts the speed-up of the communication.
Patent Literature 1 discloses a technique of controlling a communication line to be maintained in a constantly low impedance state in order to mitigate an adverse effect caused by the induced voltage.